CCNA Network Infrastructure and Connectivity Practice Question
A network engineer replaces a failed 1000BASE-LX SFP on a core switch with a new transceiver of the same type. After connecting the single-mode fiber, the link remains down and a 'show interfaces gig1/0/49 transceiver' reveals an Rx power of –30 dBm, while the far-end SFP is transmitting at –3 dBm over a 2 km span. The fiber patch cord shows no visible damage.
⚠ Common exam trap
Cisco often tests the concept that a link can fail due to excessive optical loss even when the fiber distance is well within the rated maximum, leading candidates to incorrectly blame distance or counterfeit hardware instead of connector cleanliness or damage.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.
The measured Rx power of –30 dBm is far below the receive sensitivity threshold for 1000BASE-LX (typically –19 to –22 dBm), even though the transmitter is outputting a healthy –3 dBm over only 2 km. This indicates excessive loss in the optical path, most commonly caused by a dirty or damaged fiber connector. Cleaning the connector ends with an appropriate fiber cleaning tool and inspecting with a microscope would likely resolve the issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The new SFP is a counterfeit Cisco transceiver that cannot establish a stable link.
Why it's wrong here
A counterfeit SFP may cause operational issues such as link flapping or being placed in an error-disabled state due to Cisco's transceiver validation, but it would not typically cause a drastically low optical receive power reading—the power measurement reflects actual light received.
- ✗
The SFP is not fully seated in the switch port, causing an intermittent optical connection.
Why it's wrong here
If the SFP were not fully inserted, the fiber would likely not couple light into the receiver at all, resulting in no Rx power measurement (e.g., '-Inf' or '--'), not a specific –30 dBm reading that indicates some light is reaching the photodiode.
- ✓
Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.
Why this is correct
A –30 dBm Rx power with a transmit level of –3 dBm over a 2 km single-mode span represents a 27 dB loss, far exceeding the expected 0.5–1 dB. Such high loss is typical of contaminated end faces, poor mating, or a tight bend, and it pushes the signal below the receiver sensitivity threshold (around –25 dBm), causing the link to stay down.
- ✗
The single-mode fiber distance exceeds the 10 km maximum for 1000BASE-LX, leading to severe signal dispersion.
Why it's wrong here
The fiber run is only 2 km, well within the LX specification. Even at 10 km, single-mode fiber would introduce only about 5 dB of attenuation (0.5 dB/km), resulting in a receive power of roughly –8 dBm, not –30 dBm. Dispersion does not cause a power-level drop; it distorts the signal shape.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.Correct answer▾
Why this is correct
A –30 dBm Rx power with a transmit level of –3 dBm over a 2 km single-mode span represents a 27 dB loss, far exceeding the expected 0.5–1 dB. Such high loss is typical of contaminated end faces, poor mating, or a tight bend, and it pushes the signal below the receiver sensitivity threshold (around –25 dBm), causing the link to stay down.
✗The new SFP is a counterfeit Cisco transceiver that cannot establish a stable link.Wrong answer — click to see why▾
Why this is wrong here
Low Rx power points to a physical signal issue, not a counterfeit detection problem.
✗The SFP is not fully seated in the switch port, causing an intermittent optical connection.Wrong answer — click to see why▾
Why this is wrong here
A partially seated SFP would likely prevent any light from entering, not show a measurable but weak signal.
✗The single-mode fiber distance exceeds the 10 km maximum for 1000BASE-LX, leading to severe signal dispersion.Wrong answer — click to see why▾
Why this is wrong here
Distance would not cause a 27 dB loss over such a short path, and dispersion is not measured as a reduction in optical power on the DOM readout.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Key term
SPAN
A Switch Port Analyzer (SPAN) is a feature on network switches that copies traffic from one or more ports to a monitoring port for analysis without disrupting normal network operations.
Key term
Switched Port Analyzer
A Switched Port Analyzer (SPAN) is a feature on network switches that copies traffic from one or more ports to a monitoring port for analysis.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.